commercial economy is due also to the cheap fuel which is made
possible by the cycle adopted. The engines are built in side-by-
side sets of two or three units on one shaft, each piston developing
usually 80 B.H.P., and each set of three 240 B.H.P. ; but almost
any size can be supplied from 8 to 1000 H.P. Although the
mechanical efficiency decreases on light load, the thermal efficiency
increases, so that the following results have been obtained on one
engine at various loads :
At 160 B.H.P ................ -40 Ib. oil per B.H.P. hour.
„ 84*8 ,, ............... '46 „ „ „ ,, ,,
>> 60 „ ............... -50
„ „ „
a very creditable performance.
P. 882. Graphic determination of pVn=C. The
log/ + — logV = logC
is of the form
y + mx = c
which is the equation to a straight line ; where y is the ordinate,
x the abscissa, and m the slope. It is therefore proved that log p
is the vertical ordinate, log V the horizontal abscissa, and ~
the slope of the line in the lower diagram.
P. 883. Rankine Cycle. Some authors refer to this as
the Clausius cycle, but the nomenclature of the Institution of Civil
Engineers has been adopted in this book • Fig. 848, p. 890, being
called after Rankine, while Fig. 850, p. 890, is termed the
common steam diagram.
Pp. 895 and 966. The Steam Turbine.
General considerations. This prime mover has now reached
an efficiency slightly greater than that of its rival the reciprocating
engine, and its study is therefore of some importance. The highest
economy, it may be at once stated, is only obtained i)y a combina-
tion of superb eating and condensation ; superheats of 70° F. giving
8 % increase in power in a de Laval, and 1.4 % in a Curtis turbine,